Add changelog for 2.0.0b1

[consideRatio]

• for Release planning for v2 #991

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Is there something else to include before going for a 2.0.0b1 release?

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I've recently self-merged these and included them in this changelog.

• Update traefik from 2.10.1 to 3.1.4 #1001
• Bump the requirements-user-env-extras.txt lower version bounds #1002

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Link to rendered changelog: https://the-littlest-jupyterhub--1000.org.readthedocs.build/en/1000/reference/changelog.html

I've not done work including v1 -> v2 upgrade instructions. My take is that its relevant to do when there is a distribution specific migration that needs to be done for example, but I think that isn't the case for this major version of tljh, so I've leaned on referencing other software's changelogs.

[minrk]

Thanks @consideRatio !